1. Field of the Invention
This invention relates to a sound field correction circuit for a surround acoustic system, and more particularly to simplification of a decoding circuit for obtaining multi-channel surround signals from 2-channel stereo signals encoded for a multi-channel surround effect.
2. Description of the Related Art
It is known that a human being orients low-pitched sound sources based on a time difference and/or phase difference between sounds arriving at each ear, but orients high-pitched sound sources based on the strength difference between incoming sounds to each ear.
Also, it is known that a person's sense of hearing blends sounds from a plurality of sound sources to form a single sound image. In this instance, the sounds are more likely to be blended when the signals are coherent and the arrival times of the signals are limited to a certain range. The orientation of a composite sound image, when two coherent sound sources are involved, depends upon the amplitude difference or the time difference between the sound source signals. A composite sound image from coherent sound sources appears to be substantially oriented between the two sound sources. However, when the sound sources have opposite phases, the sound image produced appears to be sourced within the listener's head. When in addition to the direct sound from a sound source, there are a large number of succeeding sounds, such as reflected sounds from the walls and floor, conveyed to the listener the expansiveness of the sound or the distance of the sound source appears to increase. Particularly, when sound reflected from the side becomes large in comparison to the direct sound, then the extensiveness of the sound image increases.
An available system, wherein the feeling of movement is emphasized using a rear speaker or the like in order to improve the presence of the played back sound, that makes use of the above-described effect is a surround system. A surround system generally is an acoustic system having, in addition to left and right speakers for providing an original stereo effect, a sub-speaker placed at a different position.
While a surround system is similar to a 4-channel stereo system, in that an additional speaker is used in addition to the left and right speakers, it is quite different. The contents of the signal recorded on an original source, such as a record or a video tape, are not special as is the case for a 4-channel stereo system. Surround system signals may be the same signals as recorded for an ordinary stereo system. In other words, the surround system does not require a specially recorded signal. The only signal criteria is the satisfaction of the listener.
Therefore, what is significant is the purpose of the person trying to enjoy the sound. Such purposes can be divided into the following two types:                1) Reproduction of an audio signal of a video tape. The purpose is a theater effect wherein played-back sound is made to realistically conform to an image; and        2) When the relationship to a picture is not taken into consideration. The purpose is to conscientiously reproduce the sound field at a site, such as a hall where the music was recorded.        
The problem is how to produce the sound so that either purpose, as described above, can be achieved. In the case of purpose 1 above, when the sound depends upon the scene, reproduction of the position of a sound source or of the feeling of movement is needed. In the case of purpose 2, where the sound source does not move and it is desired to conscientiously reproduce the acoustic conditions of the place where the sound was recorded, the length or the magnitude of reverberation and the frequency components included in the reverberation are important.
For example, in purpose 1 above, if it is assumed that the video includes a scene in which it thunders, then it is more realistic if the thunder sounds from above. To achieve this effect, greater importance needs to be attached to the position or the distance to a sound source or the feeling of movement. Reproduction from such a point of view, as described above, is basically impossible with stereo signals recorded by an ordinary method. A source where the sound was collected upon recording using some suitable discretion is more advantageous.
A representative surround system, as described above, is the Dolby pro-logic surround system. The Dolby pro-logic surround system is a modification of the Dolby surround system, which originally was an acoustic system for a movie theater, in that it employs three front channels (L: Left, C: Center, R: Right) and one rear channel (S: Surround) to make an acoustic system for enjoying a movie.
In such a multi-channel surround system as just described, multi-channel signals are normally encoded and recorded in 2 channels on a recording medium. Then, on playback, the 2-channel signals from the recording medium are decoded into multi-channels to produce multi-channel surround signals. Further, when the thus decoded multi-channel surround signals are played back into the front 2-channel surround signals of an ordinary stereo apparatus the multi-channel surround signals are converted into 2-channels by a circuit called a virtualizer.
A sound field correction circuit which first converts 2-channel audio signals into multi-channel surround signals and then into front 2-channel surround signals is shown in FIG. 1. Stereo signals Lt, Rt encoded for a multi-channel surround effect are first sent to a decoder 1. Then, the stereo signals Lt, Rt are converted from 2-channel signals back into multi-channel signals (for example, L: left channel signal, C: center channel signal, R: right channel signal, S: surround channel signal) in the decoder 1. The resulting multi-channel signals are sent to a virtualizer circuit 2. The virtualizer circuit 2 is a circuit which processes an audio signal so that the sound appears to be coming from around or from in back of the listener, as if a surround component S were actually present.
In the ordinary sound field correction circuit described above, decoding of signals from a 2-channel source into multi-channel signals is realized by performing complicated matrix processing using a decoding circuit called a surround processor. The decoding circuit is a combination of a plurality of delay circuits or phase shifters and produces a sum signal and/or a difference signal between an original signal and a signal obtained by performing a delaying process or a phase shifting process. This process enriches the extensiveness of the sound, the distance feeling for the sound image, the feeling of movement of the sound image, and so forth. Because the decoding circuit performs such complicated processes, when an ordinary decoding circuit is employed the decoding circuit significantly increases the size and the cost for the entire apparatus.
Therefore, in the ordinary sound field correction circuit for playback of surround signals as described above, when a complicated matrix process is performed by a circuit for decoding a drawback is that the circuit becomes large and consequently the apparatus becomes expensive.